The present invention generally relates to radiographic inspection techniques and, more particularly, to an alignment tool and to a radiographic inspection process.
Radiographic inspection techniques have been used for over half a century for nondestructive testing of weld, castings, and forgings in a variety of industries. Radiographic inspection techniques are currently used, for example, for the evaluation of aging and contemporary aircraft, as well as during aircraft manufacturing, maintenance, and repair. Radiographic inspection may be employed beneficially to detect hidden defects, such as cracks, gaps, and corrosion, to assess internal damage, and to detect foreign objects, for example, in airframe structures. Aircraft nondestructive inspection and evaluation eliminates the need for unnecessary maintenance and aircraft disassembly, which has the potential for creating additional damage and repair problems. High-energy radiation can be used to study the condition of aircraft structure. Gamma rays from absorbed materials and x-rays from vacuum tubes are the type of energy source used for radiographic inspection techniques. The function of an x-ray tube is to convert electrical energy into x-rays. The output of the tube is rated in kilovolts. Most aircraft tubes run approximately 150 kilovolts, a relatively modest energy level. Energy waves pass through the metal undergoing irradiation and some of the energy is absorbed in that process. The amount of absorption is dependent upon the density and thickness of the metal. The differences in the absorption are usually measured by exposure to radiographic film. The exposed radiographic film, also called radiograph, is the heart of a radiographic inspection. A radiographic film is composed of a sheet of clear cellulose or triacetate that is treated on both sides with an emulsion of gelatin and silver halide compounds. When exposed to x-radiation, gamma rays, or light, these silver halide compounds undergo a chemical change. When the exposed film is treated in a chemical solution (developer), further reaction takes place. The silver halide compounds form tiny crystals of black metallic silver. It is this silver, suspended in gelatin on both sides of the triacetate base, that forms the radiographic image. The film then resembles a photographic negative. Thinner sections of material will appear darker than thicker ones. On a radiograph, the areas most severely pitted, exfoliated, or affected by intergranular attack will appear darker than the rest of a test specimen.
The correct interpretation of the radiograph depends essentially on the image quality and the interpreter's experience in his evaluation. The image quality depends significantly on the achievement of proper alignment of the x-ray or gamma ray source with the intended condition, such as a material defect. Traditional methods to align the x-ray or gamma ray source, for example, the x-ray tube with the condition require a stepped process. During this alignment process the radiographic film is placed on the part to be irradiated, the x-ray tube is energized, the radiographic film is processed, the radiograph is evaluated, and adjustments are then made to the location of the x-tube before the next exposure. This process needs to be repeated as many times as necessary to achieve the desired image quality. Each exposure cycle takes on average 20 to 40 minutes, which is included in cycle times and reflects in manufacturing or inspection cost of the product, for example an aircraft airframe. As engineering requirements currently approach tighter defect widths, the x-ray or gamma ray source to condition alignment becomes more critical thus increasing the number of exposures needed to obtain desired image quality of the radiograph.
Furthermore, the alignment quality feedback to the interpreter of the radiograph, such as an inspector or technician, is crucial for the evaluation of the radiograph. Currently, inspectors typically measure a geometric feature of known size on the exposed and developed radiographic film to determine alignment quality. This approach creates problems due to geometric magnification, x-ray or gamma ray source to part distance, and focal spot size.
Presently available radiographic tools, such as x-ray tubes, are designed to project a laser image corresponding with the central ray emitted by the radiographic tool when energized. This laser-sighting feature is very useful to the technician in approximating alignment between the central x-ray or gamma ray and the median point of the part to be imaged. Currently the laser may not be used for precise alignment of the radiographic tool.
Prior art includes various radiographic inspection aids, for example, U.S. Pat. No. 5,402,577, issued to Cummings. The described radiographic inspection aid can be used for the radiographic inspection of electron beam welds. The described radiographic inspection aid enables one-dimensional alignment of an x-ray tube, but cannot be used as an alignment quality indicator. The disclosed prior art inspection aid is used as an angle alignment tool which assists in ensuring that x-rays aimed at welds during inspection penetrate the weld seam at an angle of approximately 90 degrees measurable against the surface of the weld along both its length and width. Even though it may be possible to use the prior art alignment aid to align an x-ray tube, it is not possible to use the prior art inspection aid to verify the alignment quality due to the poor radiographic qualities of the alignment aid.
As can be seen, there is a need for an alignment aid that eliminates the currently used stepped alignment process of the radiation source, for example, an x-ray tube, and, consequently, reduces the inspection cycle time. Furthermore, there is a need for an alignment aid that improves the alignment quality feedback to the interpreter of the radiographs and provides a permanent record of the exposure.
There has, therefore, arisen a need to provide an alignment tool that enables the accurate alignment of a radiation source used during radiographic inspection to a condition to be inspected without the need to take multiple exposures. There has further arisen a need to provide an alignment tool that enables three-dimensional alignment of a radiation source, such as an x-ray tube, to a condition that allows the detection and identification of small width defects in the irradiated part. There has still further arisen a need to provide feedback to the inspector that the alignment was correct for the exposure thus improving inspection confidence.